FIG. 1 depicts a simplified block diagram of a prior art single output linear array CCD image sensor. Image sensor 100 includes a single linear row of photodetectors 102, also known as a linear array 104. Light is converted to photo-carriers (i.e., electrons or holes) by the photodetectors 102. The photo-carriers are subsequently simultaneously transferred, as discrete charge packets 106, to shift register elements 108 in horizontal CCD shift register 110 using transfer mechanism 112. The charge packets 106 are then serially transferred or shifted to charge sensing node 114 by electrically clocking the shift register elements 108. Charge sensing node 114 typically includes a floating diffusion (not shown) that is connected to output buffer 116.
With a single output linear image sensor, charge sensing node 114 and output buffer 116 are located directly at the end of horizontal CCD shift register 110. This placement allows an un-impeded transfer of charge to charge sensing node 114 and output buffer 116. The direction of charge transfer is substantially linear through horizontal CCD shift register 110, charge sensing node 114, and output buffer 116. There is no spatial interference between horizontal CCD shift register 110 and charge sensing node 114 and output buffer 116, allowing the design of each to be optimized for speed and signal quality.
The operating speed of a linear array CCD image sensor is expressed as lines read out per second. The operating speed is typically increased by using multiple outputs. FIG. 2 illustrates a simplified block diagram of a multiple output linear array CCD image sensor in accordance with the prior art. Each output structure 200 is connected to, and senses the signal from a sub-array 202 of photodetectors, and is located at the end of each horizontal CCD shift register 204.
The problem with conventional multiple output linear image sensors is that the output structures cannot be located in close proximity to the end of each horizontal CCD shift register. The direction of charge transfer is interrupted and not linear through horizontal CCD shift register 110, charge sensing node 114, and output buffer 116. This is because the size of the output structure is much larger than the pitch of the photodetectors. And the photodetectors must be arranged in an un-interrupted array. Thus, there is a physical interference between the output structures and regular pattern of the photodetectors and horizontal CCD shift registers.
In the illustrated embodiment of FIGS. 2 and 3, it is necessary to re-direct the transfer of charge through turns or additional shift register elements that interfere in the charge transfer process. These turns increase the distance that the charge must be transferred to reach the output structure, as shown by the arrows in FIG. 3. Each shift register element 300 includes four phases 302 and charge transfer is between each phase in the FIG. 3 embodiment. The charge transfer distance 304 from the last phase to the output is much larger than the charge transfer distance 306 between the phases of the horizontal CCD shift register. The longer charge transfer distance 304 results in a reduced electric field to shift charge. The smaller electric field in turn causes charge transfer inefficiency or the incomplete transfer of charge to the output structure. Additionally, the smaller electric field produces a distortion of the output by mixing the signals of adjacent pixels together. This transfer inefficiency can also result in a loss of part of the signal for the first pixel of each sub-array output structure.